The subject matter disclosed herein relates to refrigeration systems. More specifically, the subject matter disclosed herein relates to refrigeration of containers utilized to store and ship cargo.
A typical refrigerated cargo container or refrigerated truck trailer, such as those utilized to transport a cargo via sea, rail or road, is a container modified to include a refrigeration unit located at one end of the container. The refrigeration unit includes a compressor, condenser, expansion valve and evaporator serially connected by refrigerant lines in a closed refrigerant circuit in accord with known refrigerant vapor compression cycles. The evaporator is located in a compartment that requires cooling, such as a cargo compartment of a truck or trailer. The condenser and compressor are located outside of the compartment. Cargo compartment air is passed over the coils of the evaporator, boiling the refrigerant flowing through the evaporator coil, thus heat is absorbed from the air in the conditioned compartment to cool the conditioned compartment. The gaseous refrigerant is then flowed to the compressor for compression thereat. A power unit, including an engine, drives the compressor of the refrigeration unit, and is typically diesel powered, or in other applications natural gas powered. In many truck/trailer transport refrigeration systems, the compressor is driven by the engine shaft either through a belt drive or by a mechanical shaft-to-shaft link. In other systems, so-called “electrically driven” systems, the engine drives a generator that generates electrical power, which in turn drives the compressor.
Refrigerant R404A is popularly used in refrigeration and transport refrigeration applications, but is being phased out in many locations due to its high global warming potential (GWP). Potential replacement refrigerants for R404A, such as R407F or R448A, have shown equivalent performance to R404A at high and medium temperature refrigeration points (about 30 degrees Fahrenheit and above), but typically have 10-15% lower capacity than R404A at frozen goods refrigeration temperatures (about −30 degrees Fahrenheit). Additionally, the replacement refrigerants show higher compressor discharge temperatures compared to R404A.
Additional compressor displacement or speed is typically used to overcome the cooling capacity shortfall. Utilizing a single, larger compressor, and/or operating it at higher speeds decreases efficiency of the refrigeration system. Additionally, larger compressors often need to be throttled to maintain the high and medium refrigeration temperatures, at which point their performance worsens.